A conventional pulse-excited laser oscillator is described in "Volume 1, Solid-State Laser Engineering" by Walter Koechner in "Springer Series in Optical Sciences" published by Springer-Verlag New York Inc. in 1976. The laser oscillator comprises a charging unit including capacitors by which a predetermined voltage is applied to a flash lamp, and a trigger circuit which generates a driving voltage to be applied to a laser medium in accordance with the discharge current flowing in the flash lamp.
In operation, output energy of the laser oscillator is controlled to be a predetermined level in accordance with the amount and time duration of the discharge current flowing in the flash lamp. For this purpose, the charged voltage of the capacitors is precisely controlled such that the discharge current flowing in the flash lamp is maintained to be a predetermined value.
In the laser oscillator mentioned above, however, a constant output energy is difficult obtain because the output energy decreases in accordance with the decrease of light radiating from the flash lamp due to the deterioration of the flash lamp even if the charged voltage of the capacitors is maintained constant. In addition, in a case where repetition laser oscillation is performed, the output energy varies depending upon the repetition frequency of driving pulses because the temperature distribution varies in the laser medium dependent upon that frequency because the laser medium is heated by the flash lamp so that the change of light axis is induced in the laser medium even if the charged voltage is maintained constant so that the light amount of the flash lamp is of a predetermined level.
Another conventional pulse-excited laser oscillator is described in Japanese Utility Model laid open No 60-76050 (76050/1985). The laser oscillator comprises a power monitoring circuit including a PIN photodiode for converting laser light output to electric signal, an integrating circuit for integrating the output of the power monitoring circuit to produce a mean value thereof, a laser output controlling circuit for comparing the output of the integrating circuit with a predetermined laser output value, a feedback controlling circuit for comparing the discharge current or voltage of a flash lamp (to which the output of the laser output controlling circuit is added) with a predetermined discharge current or voltage, and a SCR controller which is controlled in accordance with the output of the feedback controlling circuit.
In operation, pulses which are output from the power monitoring circuit are integrated in the integrating circuit to output a mean value thereof. The mean value is compared in the laser output controlling circuit with the predetermined laser output value to output a subtracted signal of the two inputs thereto. The subtracted signal is added to the discharge current or voltage of the flash lamp whereby the added value is compared in the feedback controlling circuit with the predetermined discharge current or voltage to output a control signal. The SCR controller is controlled dependent upon the control signal to control a driving circuit so that a predetermined level of laser light output is obtained.
In the latter laser oscillator, however, a stable light output is not obtained in each pulse in real time because the pulses converted from laser light output in the power monitoring circuit are integrated in the integrating circuit thereby to produce a mean value of the pulses. Further, the laser light output and discharge current or voltage of the flash lamp are simultaneously sampled to control a laser oscillation so that the construction thereof and control sequence become complicated.